What is the difference between those (apart from the fact that REST is free)?

I am now getting my Google Analytics web data via Google Sheets which is not comfortable to use since it works slow (many rows of data) and requires separate sheet for each combination of Dimensions ans Measures.

Therefore I want to establish direct connection to GA and trying to figure out which connector to use. Web Connector is quite expensive for my company, so i consider REST connector to work with.

basically the only (but major) difference is that GA connector provides you with pre-built requests, so you can get up and running very fast by copy/paste of script from GA connector do Qlik Sense, plus maintainance of such code is quite OK. With REST connector you have to build all your requests by yourself and if your requirements change and you need to change those requests, well, goo luck with that.